1. Field of the Invention
The present invention relates generally to a Bandwidth (BW) Request (REQ) channel in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system, and more particularly, to transmission, reception and modulation methods of a BW REQ channel used by a Mobile Station (MS) to request an uplink BW from a Base Station (BS).
2. Description of the Related Art
Conventional OFDMA systems utilize a multiple access scheme in which the BS schedules and allocates time/frequency/space resources to multiple MSs using a scheduler. When using the multiple access scheme, it is necessary for the BS scheduler to strictly manage the resources in order to prevent a collision of resources in the OFDMA channel. When an MS generates uplink traffic for transmission to the BS, the MS is first required to request that the BS allocate bandwidth resources for the transmission of the uplink traffic. When an MS has not yet been allocated an uplink resource when a request is required to be made, a resource that is accessible by every MS is typically used to request the uplink bandwidth.
An example of a BW REQ signaling procedure in an IEEE 802.16e system is illustrated in FIG. 1. The procedure begins at step 102, in which the MS randomly selects one of a plurality of BW REQ ranging codes, and transmits the selected ranging code to the BS using the time/frequency/space resource previously allocated to the BW REQ channel or the resource accessible by every MS. In step 104, the BS detects the transmission of the BW REQ ranging code, and determines whether the transmitted BW REQ ranging code is one of the BW REQ ranging codes that are transferable in the BW REQ channel. Upon a positive determination of the BW REQ ranging code, the BS allocates an uplink resource of a pre-defined size to the MS based on a broadcast identifier (ID). The size of the allocated uplink resource is pre-defined between the BS and the MS according to the BW REQ ranging code.
In step 106, when the uplink resource is allocated by the BS in response to the BW REQ ranging code, the MS transmits BW REQ information including a corresponding MS-ID, or alternatively a Connection or Station ID (CID or SID), and a requested uplink resource corresponding to a bandwidth required to deliver the uplink traffic in a BW REQ message. The BW REQ message is transmitted according to a designated format, such as a BW REQ header. Accordingly, it is necessary for the uplink resource allocated in step 104 to have a pre-defined size capable of delivering the designated format.
In step 108, the BS acquires the BW REQ message, including the MS-ID (or the CID or SID) and the requested uplink resource by decoding the BW REQ header. The BS reflects the BW REQ information in an uplink scheduler and grants another uplink resource to the MS in accordance with a size requested by the MS at a point in time permitted by the scheduler.
In step 110, when the uplink resource is granted, the MS transmits the uplink traffic that was stored in a queue of the MS. When additional uplink resources are required, the MS can piggyback BW REQ message to the allocated resource without having to repeat the BW REQ signaling procedure from step 102.
The conventional BW REQ signaling procedure described above requires five steps of signaling between the MS and the BS, and is commonly referred to as the five-step BW REQ signaling procedure. Each signaling step requires encoding/decoding and contention resolution, thereby requiring more than one frame. Thus, it is difficult for the five-step BW REQ signaling procedure to support delay sensitive Real-Time (RT) services, which require transmission as soon as possible after the uplink traffic is generated.